Process for producing superabsorbent particles

Abstract

A process for producing thermally surface postcrosslinked superabsorbent particles, comprising polymerization of a monomer solution or suspension, static drying of the resultant aqueous polymer gel, comminution of the dried polymer gel, classification of the resultant polymer particles, with removal of excessively small polymer particles as undersize, mixing of the removed undersize with an aqueous solution, said aqueous solution comprising a crosslinker, and recycling of the polymer gel obtained from the undersize into the static drying.

1 . A process for producing surface postcrosslinked superabsorbent particles by polymerizing a monomer solution or suspension comprising
 a) at least one ethylenically unsaturated monomer which bears an acid group and is at least partly neutralized,   b) at least one crosslinker 1, and   c) at least one initiator,   comprising   i) polymerizing the monomer solution or suspension and optionally extruding the resultant polymer gel 1,   ii) drying the polymer gel 1,   iii) comminuting the dried polymer gel 1,   iv) classifying the polymer particles obtained in step iii), with removal of excessively small polymer particles as undersize 1 and optionally with recycling of excessively large particles into step iii), and with thermal surface postcrosslinking of the remaining polymer particles in a further step,   v) mixing the removed undersize 1 with an aqueous solution and optionally extruding a resultant polymer gel 2, and   vi) recycling the polymer gel 2 into step ii),   wherein the polymer gel is statically dried in step ii), the moisture content of the polymer gel 2 obtained in step v) is from 20% to 80% by weight, the aqueous solution in step v) comprises at least one crosslinker 2, and the crosslinker 2 can form covalent or ionic bonds with at least two carboxylate groups of the polymer particles.   
     
     
         2 . The process according to  claim 1 , wherein, in step v), the removed undersize 1 is first mixed with water and/or an aqueous base and optionally extruded and then mixed with the aqueous solution and optionally extruded. 
     
     
         3 . The process according to  claim 1 , wherein the aqueous solution in step v) comprises from 0.01% to 1.0% by weight of the at least one crosslinker 2, based on the amount of removed undersize. 
     
     
         4 . The process according to  claim 1 , wherein aqueous solution in step v) comprises from 0.05% to 0.2% by weight of the at least one crosslinker 2, based on the amount of removed undersize. 
     
     
         5 . The process according to  claim 1 , wherein the at least one crosslinker 2 used in step v) can form covalent bonds with at least two carboxylate groups of the polymer particles. 
     
     
         6 . The process according to  claim 5 , wherein the at least one crosslinker 2 used in step v) comprises at least two epoxy groups. 
     
     
         7 . The process according to  claim 1 , wherein a moisture content of the polymer gel obtained in step v) is from 40% to 60% by weight. 
     
     
         8 . The process according to  claim 1 , wherein 90% by weight of the undersize 1 removed in step iv) has a particle size not exceeding 250 μm. 
     
     
         9 . The process according to  claim 1 , wherein 90% by weight of the undersize 1 removed in step iv) has a particle size not exceeding 150 μm. 
     
     
         10 . The process according to  claim 1 , wherein an amount of polymer gel recycled into step vi) is from 1% to 50% by weight, based on the total amount of polymer gel to be dried in step ii). 
     
     
         11 . The process according to  claim 1 , wherein an amount of polymer gel recycled into step vi) is from 20% to 30% by weight, based on the total amount of polymer gel to be dried in step ii). 
     
     
         12 . The process according to  claim 1 , wherein a dwell time of the polymer gel 2 between steps v) and ii) is not more than 5 minutes. 
     
     
         13 . The process according to  claim 1 , wherein a temperature in the drying operation in step ii) is at least 120° C. 
     
     
         14 . The process according to  claim 1 , wherein a dwell time in the drying operation in step ii) is at least 10 minutes. 
     
     
         15 . The process according to  claim 1 , wherein the surface postcrosslinked polymer particles are classified, with removal of excessively small polymer particles as the undersize 2, recycling of the removed undersize 2 into step v), where a mass of the undersize 2 in a total mass of undersize is not more than 10% by weight.